<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 500px;
            height: 500px;
            background-color: royalblue;
        }
        .inner{
            width: 250px;
            height: 250px;
            background-color: skyblue;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="inner"></div>
    </div>
    <script>
        var oBox=document.querySelector(".box");
        oBox.onclick=function(evt){
            var e=evt||window.event;
            console.log(e.target);
            // var ele=e.target||e.srcElement;
            // console.log(ele);
        }

        document.onmousedown=function(evt){
            var e=evt||window.event;
            console.log(e.button);
        }
    </script>
</body>
</html>